In order to generate X-ray radiation in an imaging medical facility, (e.g., in a computed tomography scanner), X-ray tubes are used. In the X-ray tube, a heated cathode emits electrons that are accelerated by a high voltage of 20 kV-200 kV towards an anode. On impinging on the anode, the electrons enter the anode material. In this case, two types of X-ray radiation may be generated. As a result of energy conservation, the bremsstrahlung is emitted as a continuous spectrum on deceleration of the electrons in the anode material. Furthermore, the electron beam ionizes atoms in the anode material, which results in transfers of fixed energy levels and, as a result, emission of the characteristic X-ray radiation takes place. This has a linear spectrum. In medical applications, the bremsstrahlung is used widely so that efficient manipulation of this radiation plays a key role.
Since the X-ray with conventional optical components may not be oriented in optimum fashion as a result of the short wavelengths, the electron beam may be focused and directed to a specific point on the anode surface. The properties and orientation of the anode and the anode material used in this case have an effect on the direction and the ray profile of the generated X-ray. In particular, the position of the focal spot, (e.g., the zone in which the electron beam impinges on the anode), may in this case be mentioned as an important control parameter.
In order to deflect the electron beam, but also to focus said electron beam, magnetic fields may be used. The magnetic fields are generated by deflection coils, for example, which are arranged between the cathode and the anode around the electron beam or the X-ray tube. Depending on the anode geometry and on requirements for the sharpness of the focusing or for the focal spot contour, one or more deflection coils may be provided.
In this case, a current in the respective deflection coils is varied as manipulated variable for the position and the profile of the electron beam. Only the so-called flying focal spot method, which is characterized by targeted periodic shifting of the focal spot, may include a rectangular current profile in the respective deflection coil, which rectangular current profile has a high rate of rise in the edges and as small fluctuations as possible in the plateau.
A circuit including a full bridge or a half bridge may be used for this purpose, the circuit being connected at its input terminals to a DC voltage circuit, wherein the respective deflection coil is interconnected, for example, between the two output terminals of the full bridge or from an output terminal of the half bridge to ground. For current regulation, pulse width modulation may be used. In order to generate a current pulse that is as rectangular as possible in the deflection coil, the DC voltage source is applied to the deflection coil until the desired current is reached, and the voltage is then switched over to another value.
For the desired rate of rise of the current in the edges, a voltage in the DC voltage circuit that is as high as possible is provided. Whilst maintaining the current, the high voltage applied in pulsed form generates fluctuations in the current flow, however, which in turn result in fluctuations in the magnetic flux through the deflection coil. However, these fluctuations are undesirable since, as a result, the quality of the focusing or positioning of the electron beam and therefore ultimately the quantity of the X-ray deteriorates.